Gripping device for manipulating device and method for operation thereof

ABSTRACT

A gripping device for a manipulation system, particularly a robot, for receiving workpieces and feeding them from a readied stack of the workpieces to a manufacturing plant such as a metal sheet folding machine, punching press, welding plant, etc. The device has a gripper head fitted with gripping means such as suction cups, magnets, tongs, etc., and a detection system for detecting characteristics of the workpiece gripped by the gripping means. A pulse emitter excites vibrations in the workpiece and the vibration spectrum of the workpiece is compared to reference vibration data to determine characteristics of the gripped workpiece, such as whether two or more workpieces are stuck together, or whether the workpiece is the correct workpiece.

BACKGROUND OF THE INVENTION

The invention relates to a gripping device for a manipulation systemcomprising a robot for receiving parts and feeding a manufacturing plantwith a workpiece from a readied stack of workpieces. The invention alsorelates to a method for operating the gripping device.

In the manipulation of workpieces with automated manipulation systems,so-called robots, where the workpieces are seized and lifted by means ofa gripping device from a made-ready position and then have to be fed toa manufacturing system where they are worked, problems often occur withthe feed from a stack of cut or punched flat workpieces such as metalsheets, for example due to surface contamination caused by a film ofcutting or punching oil, causing the sheets to adhere to each other, sothat two or more sheets are picked up from the stack by the grippingdevice, for example a suction or magnetic gripper, instead of only onesingle sheet, leading to interference with the production process.

Possibilities for remedying such a situation are known in the prior art,one such possibility being to equip the gripping device with a weightsensor in order to determine the weight of the seized sheets lifted onthe gripping device with the help of the parameters stored in acomputer, and to then separate the workpieces, if necessary.

Another possibility offered by the prior art is to carry out an opticalmeasurements, for example of the thickness after the sheet has beenseized, and to then determine the number of workpieces seized based onthe weight with the help of the parameters stored in a computer, inorder to subsequently separate the workpieces, if need be.

Furthermore, ultrasound, eddy current and magnetic measuring methods areknown, by which the total weight is determined, and the further courseof action is then determined as in connection with an opticalmeasurement.

Moreover, it is known also to divide the workpieces into single piecesbefore they are seized if such pieces stick together. However, thedevices required for such separation require high expenditure in termsof mechanical components, and their operation is connected withincreased controlling expenditure.

SUMMARY OF THE DISCLOSURE

Now, the problem of the invention is to offer a gripping device for amanipulation system that permits quick recognition of the number ofworkpieces picked up by the gripping device, and which can be realizedwith a low weight and compact design.

The invention addresses this problem by providing a gripping devicecomprising a gripper head supporting gripping means, and a detectionsystem for detecting a workpiece received by the gripping means, andcomprising at least one pulse emitter acting upon the workpiece toexcite vibrations in the workpiece, and at least one vibration sensorfor sensing the vibrations of the workpiece, and a memory and/oranalytical module structured and arranged to conduct a vibrationanalysis on a vibration signal from said vibration sensor, wherein thedetection system and the memory and/or analytical module jointly form acomponent part detachably arranged on the gripper head and incommunication with a controller of the manufacturing plant. Thesurprising benefit gained with such features is that in addition to thecontrol as to whether the workpiece seized is a single part or a numberof parts, a further control possibility is given in that it is possibleto determine via the vibration behavior whether the seized workpiece isthe correct part, because the vibration is dependent upon both thematerial and the dimension, so the correct conclusions can be drawn inthis way as well. Furthermore, the control is carried out directly onthe gripper head, which permits dispensing with additional traveldistances of the manipulation system and to save cycle time, and higherproductivity of the manufacturing system is achieved.

However, an embodiment in which the pulse emitter is provided with apiezo sensor is feasible as well because the data decisive for avibration analysis on the seized workpiece are determined directly onthe excitation source based on the excitation pulse, which avoidsinterfering influences and permits using for the analysis a smallervibration bandwidth for the basic data, which in turn increases theprocess safety.

In one embodiment, the frequency spectrum of the vibration in theworkpiece is evaluated by comparison with stored data directly on thegripper head, so that no large amounts of data required for an analysiswill not additionally load the communication system and in particular abus system available for control measures of the manipulation system andthe gripper head.

Connecting the memory and/or analytical module with a controller of themanufacturing plant via a bus system or cable line permits a highermemory and computer capacity.

An embodiment wherein data are wirelessly transmitted between thevibration sensor and/or the memory and/or analytical module and/or thecontroller is beneficial as well in that it dispenses with electricalline connections.

Furthermore, an embodiment wherein the pulse emitter is formed by astriking tappet acted upon by kinetic energy is advantageous in that itoffers an interference-proof excitation source for generating vibrationin the workpiece.

By providing the vibration sensor as an acceleration sensor arranged tobe placed onto a surface of the workpiece, it is possible to employ asensor element that has been successfully used for high applicationfrequency.

Another advantageous embodiment has the pulse emitter and the vibrationsensor arranged together, which permits the realization of a compactdesign and where the excitation source and measurement source act via acontact point and the gripper geometry thus has no influence of theresult of the analysis.

An efficient communication and energy supply system is obtained bysupplying the detection system with power by means of the ASi bus,and/or by transferring data between the detection system and thecontroller via the ASi bus.

Furthermore, by arranging the pulse emitter and vibration sensor on thegripper head with a spacing between them, and/or by arranging contactpoints of the pulse emitter and vibration sensor on a surface of theworkpiece outside of the surface area of the workpiece defined by thegripping means, any influence of the vibration frequency due to holdingforces of the gripping means is avoided.

A further embodiment in which the detection system with the memoryand/or analytical module is detachably connected with the gripper headvia coupling means is beneficial in that it offers a self-sufficientdetector system that can be selectively fitted on gripper heads withdifferent configurations.

BRIEF DESCRIPTION OF THE DRAWINGS

For the sake of better understanding, the invention-s is explained ingreater detail in the following with the help of the exemplifiedembodiments shown in the drawings, in which:

FIG. 1 is a schematic representation of the gripper device as defined bythe invention.

FIG. 2 is a possible arrangement on a gripper head.

FIG. 3 is a vibration pulse diagram.

FIG. 4 is another embodiment of the gripping device as defined by theinvention; and

FIG. 5 is another design of the gripping device as defined by theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

It is noted here by way of introduction in the different embodimentsdescribed in the following, identical components are provided withidentical reference number or component designations, whereby thedisclosures contained throughout the specification are applicable in thesame sense to identical components and identical component designations.Furthermore, positional data selected in the specification such as, forexample “top”, “Bottom”, “laterally” etc. relate to the directlydescribed and shown figure, and have to be applied to the new positionwhere a position has changed. Moreover, individual features andcombinations of features in the different exemplified embodiments shownand described herein may represent independent inventive solutions orsolutions as defined by the invention.

FIG. 1 is a schematic representation of a gripping device 1 forreceiving the workpieces 2 from a stack 3 of such workpieces,particularly for the metal sheet workpieces 2. As shown, the grippingdevice 1 is secured on an arm 4 of a manipulation system 4, e.g. a robotnot shown in any greater detail. A gripper head 6 is fitted with thegripping means 7, for example with the suction cups 8, which areconnected via a cable line with a vacuum generator 8, whereby it isnaturally possible also to employ magnets or tongs or the like. Forlifting the workpiece 2 from the stack 3, the gripping device ispositioned with respect to the stack 3 in a preset gripping position bymeans of a controlling device 10 of the manipulation system 5, andplaced with the suction cups 9 or other gripping means on the surface 11of the workpiece 2. If suction cups are employed, the workpiece isseized by application of a vacuum in the vacuum generator 8, and liftedfrom the stack 3 in order to be fed to a manufacturing system by meansof the manipulation system 5 for a manufacturing process to be carriedout, for example by folding, punching, welding etc.

The problem often occurring in this connection is that due to a film ofoil on the surface 11 of the workpieces 2 stored in the stack 3, theworkpieces adhere to each other, so that two or more of the workpieces 2are jointly lifted from the stack as shown in FIG. 1.

For resolving this problem, the gripping device 1 as defined by theinvention comprises a detection system 12 consisting of a pulse emitter13 and a vibration sensor 15 arranged with a spacing 14 from the pulseemitter. In the exemplified example show, the pulse emitter is formed byan impact tappet 17, which is actuated by an electromagnet 16. Saidimpact tappet is controlled by the controlling device 10 in order toexcite by means of an impact pulse a vibration in the workpiece 2. Thevibration sensor 15 provided for detecting the vibration is, for examplean acceleration sensor 18, which is placed with a preset force onto theworkpiece seized by the gripping device 1 by means of a contact-pressureexerting device 19. The acceleration sensor 18 is connected by cablewith a memory module and/or analytical module via a bus system 21. Inconnection with the exemplified example shown, the memory module and/oranalytical module is an external computer, which is preferablyintegrated in the controlling device 10. It is noted that the datatransmission naturally may take place wirelessly as well.

However, an embodiment is possible also where the memory and analyticalmodule 20 is directly arranged on the gripper head 6 in order todirectly evaluate the data determined by the acceleration sensor 18 onsite, so that the bus system 21 is relieved and the data transmitted tothe controlling device 10 via the bus system 21 for the evaluation as towhether a single or more of the workpieces 2 have been picked up can belimited to the information pulses “Yes/No”.

The process for the recognition as to whether one single or more of theworkpieces 2 have been picked up is described in the following.

After the workpiece 2 has been lifted from the stack 3, the pulseemitter 3 is controlled, which, with its striking tappet 17, strikes thesurface 11 of the workpiece 2 with a minimum of contact time, andthereby puts the workpiece 2 into vibration. The vibration spectrum isrecorded by means of the acceleration sensor 18 and processed in thememory and/or analytical module 20 by means of Fourier transformation,and compared with a vibration spectrum stored for the workpiece 2 in thememory and/or analytical module 20. This reference curve of thevibration curve is determined on a workpiece in an acquisition process,or determined for a preset number of workpieces 2 in a so-calledteach-in process carried out in front-end equipment, and the data of thereference curves are stored in the memory and/or analytical module 20.There is no need for explaining in greater detail that the vibrationbehavior of the workpiece 2 is influenced by the material, thedimensions and the gripping position of the gripping means, and quitesignificantly, of course, whether only one single workpiece 2 or severalof such workpieces have been seized by the gripping device 1.

It is naturally possible to store in the memory and/or analytical module20 the processed data of the respective vibration spectrum for differentworkpieces in a workpiece matrix depending on the memory capacity, andto call in such data prior to the processing of the respective workpiece2 via a code, and to then access such data for the analysis. This permitquick retooling of a manufacturing plant and thus increases its capacityand the economy of such a production facility.

Now, FIG. 2 shows by a simplified representation a, for examplerectangular precut of the workpiece 2, the gripping means 7, the pulseemitter 13 and the vibration sensor 15 on the gripper head 6. The pulseemitter 13 and the vibration sensor 15 are arranged with the largestpossible spacing from each other for a safe analytical result.Furthermore, said components should be placed onto the workpiece 2 in asurface area 23 of the surface 11 that is located as far outside of thearea defined by the gripping means 7 as possible that would have adamping effect on the propagation of the vibration. In particular, animagined line of connection 24 between the pulse emitter 13 and thevibration sensor 15 should not extend through or between the contactpoints 25 of the gripping means 7.

Furthermore, it is of decisive importance that a pulse emitted by thepulse emitter 13 onto the workpiece 2 impacts the latter within acontact time of about 200 ms in order to prevent any damping effect.Moreover, the pulse sensor 15 should be placed by the contactpressure-exerting device 19 (described in connection with FIG. 1)against the surface 11 with a uniform contact pressure that is not beinfluenced by the vibrations. The gripping range as defined, for exampleby the contact points 25 of the gripping means 7, has to be positionedwithin narrow tolerances with respect to the references formed by theedges of the work-piece, whereby such a requirement is to be met in anycase also in view of the positioning accuracy for subsequent processingoperations, and is achieved also with the manipulation systems currentlyin use. Furthermore, it is pointed out herewith that according to anadvantageous embodiment, the detection system 12 (see FIG. 1) and thepulse emitter 13, the vibration sensor 15 and the memory and/oranalytical module 20 (see FIG. 1) preferably jointly form one componentpart, which is detachably arranged on the gripper head 6 and thereforeexchangeable, and in contact via the bus system with other componentssuch as, for example sensors, an optical positioning system etc. forsupplying such components arranged on the gripper head 6 with energy andfor the purpose of transmitting data.

Now, the diagram of FIG. 3 shows by a simplified form of curves thesignal energy of the vibration sensor from the time of vibrationexcitation until the vibration fades out. Said signal energy serves asthe basic quantity in the stored evaluation algorithm in the memoryand/or analytical module 20. Now, the differences between the curves ofthe signal energy are shown with the help of two curves determined inpractical tests, whereby the curve line 26 reflects the signal energy atthe vibration of one single workpiece seized by the gripping device 1,and the curve line 27 the signal energy at the vibration of twoworkpieces 2 sticking together. It was possible to determine with thehelp of measurement series that said curves move within a narrowbandwidth range, and that because of the significant differences betweenthe curves, a clear evaluation is achieved even when taking into accountfactors that cannot be influenced.

The diagram shows that a clear distinction is possible, and thatsuitable measures can be implemented such as the separation ofworkpieces adhering to each other. Such measures can be carried outwithout time delay directly after the vibration has been evaluated.

FIG. 4 shows a schematic representation of another embodiment of thegripping device 1. In such an embodiment of the gripping device 1, thegripper head 6 has the gripping means 7, for example in the form of thesuction cups 9. The gripper head 6 is secured on the arm 4 of themanipulation system 5 not shown in detail. Furthermore, the pulseemitter 13 is arranged on the gripper head 6 and connected for wirelesscommunication, for example via a transmitting and receiving module 28with another transmitting and receiving module 29 of the memory and/oranalytical module 20. For supplying the pulse emitter 13 with energy,the latter is connected with the bus system 21 via a cable line 30. Thememory and/or analytical module 20 is also in communication with thecontrolling device 10 via a cable line 31, or wirelessly connected tosaid controller 10.

The pulse emitter 13 has an impact tappet 17 that strikes the workpiece2 seized by the gripping device 1 with a preset striking pulse with apreset energy. Furthermore, the vibration exciter 13 is provided sensorelement 33, for example a piezo sensor 34. Said sensor element 33 servesfor determining the acceleration of the striking tappet 17 for impactingthe workpiece 2 and for determining the delay after the pulse has beenapplied.

The acceleration and delay data so determined are evaluated in acomputer module 35 of the memory and/or analytical module 20,particularly a μ-controller; the determined acceleration and delay dataare compared with reference data stored in a memory module 36, andfollowing such comparison, it is possible to determine whether thegripping device 1 has picked up one single workpiece 2 or two or moreworkpieces 2 adhering to each other via an oil film 30 or a cutting burretc., because a proportional or relative number formed based on theacceleration and delay of the striking tappet 17 is forming a clearanalytical result.

In order to further refine the analytical results, it is possible,furthermore, to additionally equip the gripper head 6 equipped with thesensor element 33 as shown by dashed lines with the vibration sensor 15described already above. For the evaluation as to whether only onesingle workpiece 2 or several of such workpieces have been picked up,both the result of the pulse analysis and the result of the vibrationanalysis are used in order to achieve high analytical safety.

FIG. 5 shows another embodiment of the gripping device 1 as defined bythe invention. In the present embodiment, the gripper head 6 is fittedwith the suction cups 9, to which a vacuum is admitted for receiving aworkpiece 2, such suction cups forming the gripping means 7 in thepresent embodiment. By means of the arm 4 of the manipulation system 5,the suction cups 9 are placed on the surface 11 of the workpiece 2readied on the stack 3, and the latter is lifted from the stack 3 byapplying the vacuum. At least one of the gripping means 7, in theconcrete exemplified embodiment two of such means are supported on thegripper head 6 via the pressure sensors 38 for determining the force(pressure or tensile force) caused by the weight of the workpiece 2.

As the workpiece 2 is being lifted from the stack 3, values measured forthe force are recorded by the pressure sensors 38 and transmitted viathe measurement lines 39 to the memory and/or analytical module 20, andcompared with stored weight data of the work-piece intended for furtherprocessing. It is possible to determine in this way whether the grippingdevice 1 picked up one single workpiece 2 or two or more workpieces 2 asshown by dashed lines, such workpieces sticking together due to, forexample an oil film 37.

Furthermore, by comparing the data measured and supplied by the pressuresensors 38 as a result of the pressure forces occurring due to theweight of the workpiece 2 as indicated by the arrow 40 of at least twogripping means 7 acting on the workpiece 2, it is possible to determinewhether the latter was gripped in the correct position in relation to areference position. This is achieved via the position determinedaccording to the force components or via a spacing 42 of the workpiece 2with respect to the reference position.

As already described above, the memory and/or analytical module 20 isnaturally connected via the cable line 31 with the controlling device 10and the computer 22, and comprises the memory module 36. However, asstated above, wireless communication is possible as well.

FIG. 5 shows, furthermore, that the separation of a number of workpieces2 adhering to each other is possible by arranging a gripping means 7that is adjustable in the direction indicated by the double arrow 43,i.e. perpendicularly to the surface 11 of the workpiece 2. For thispurpose, a center gripping means 7, for example, is supported on thegripper head 6 and adjustable by means of a servo-drive 44, for examplea pressure cylinder, to which a medium can be admitted. Such anarrangement permits reshaping the workpieces 2 from a stretched into acurved position, whereby the adhering workpiece 2 is not completelyjoining in such reshaping and a fan-like separation will occur in themarginal areas, and the adhering workpiece will finally detach itselfdue to the reduction in the auction effect as air enters into theinterface area between the two workpieces.

In the interest of good order, it is finally noted that for the sake ofbetter understanding of the structure of the gripping device, the latteror its components are partly shown untrue to scale and/or enlargedand/or reduced.

Furthermore, it is noted that the individual embodiments shown in FIGS.1, 2, 3; 4; 5 form the object of independent solutions as defined by theinvention. The relevant problems and solutions as defined by theinvention are specified in the detailed descriptions of said figures.

1. A gripping device for a manipulation system comprising a robot forreceiving parts and feeding a manufacturing plant with a workpiece froma readied stack of workpieces, the gripping device comprising a gripperhead supporting gripping means, and a detection system for detecting aworkpiece received by the gripping means, and comprising at least onepulse emitter acting upon the workpiece to excite vibrations in theworkpiece, and at least one vibration sensor for sensing the vibrationsof the workpiece, and a memory and/or analytical module structured andarranged to conduct a vibration analysis on a vibration signal from saidvibration sensor, wherein the detection system and the memory and/oranalytical module jointly form a component part detachably arranged onthe gripper head and in communication with a controller of themanufacturing plant via a bus system comprising an ASi bus.
 2. Thegripping device according to claim 1, wherein the pulse emitter has animpact tappet that strikes the workpiece seized by the gripping devicewith a preset striking pulse with a preset energy and the pulse emitteris provided with a piezo sensor for determining the acceleration of theimpact tappet impacting the workpiece and for determining the delayafter the pulse has been applied.
 3. The gripping device according toclaim 1, wherein data are wirelessly transmitted between the vibrationsensor and/or the memory and/or analytical module and/or the controller.4. The gripping device according to claim 1, wherein the pulse emitteris formed by a striking tappet acted upon by kinetic energy.
 5. Thegripping device according to claim 1, wherein the vibration sensor isformed by an acceleration sensor arranged to be placed onto a surface ofthe workpiece.
 6. The gripping device according to claim 5, wherein theacceleration sensor is supported on the gripper head via acontact-pressure-exerting device.
 7. The gripping device according toclaim 1, wherein the pulse emitter is provided with the vibrationsensor.
 8. A method for feeding workpieces from a stack of workpieces toa metal sheet folding machine for reshaping the workpieces by foldingwith a manipulation system, comprising the steps of: gripping aworkpiece at a top of the stack of workpieces using a gripping device;lifting the gripped workpiece up from the stack with the grippingdevice; exciting vibrations in the lifted workpiece using a pulseemitter arranged on the gripping device and acted upon by a controller;sensing the vibrations in the workpiece using a vibration sensorarranged on the gripping device; recording signals from the vibrationsensor in a memory and/or analytical module; and comparing a vibrationspectrum of the workpiece stored in said module with reference vibrationdata; wherein the vibration sensor is applied to a surface of theworkpiece by a contact pressure-exerting device, whereupon a pulse isapplied to the workpiece by the pulse emitter with a contact time ofabout 200 ms for exciting vibrations.
 9. The method of claim 8, whereinthe comparing step comprises comparing the vibration spectrum withreference data so as to determine whether one or more additionalworkpieces is/are stuck to the workpiece gripped by the gripping device.10. The gripping device according to claim 1, wherein the memory and/oranalytical module is structured and arranged to record signals from thevibration sensor, and to compare a vibration spectrum of the workpiecewith reference vibration data so as to determine whether one or moreadditional workpieces is/are stuck to the workpiece gripped by thegripping device.
 11. The gripping device according to claim 1, whereinthe memory and/or analytical module is structured and arranged to recordsignals from the vibration sensor, and to compare a vibration spectrumof the workpiece with reference vibration data so as to determine viathe vibration spectrum whether the seized workpiece is the correct part.